NASA’s Perseverance rover has discovered several thousand small, light-toned rock fragments, or float rocks, with some exhibiting spectral signatures of an aluminum-rich clay mineral called kaolinite. To interpret their formation, planetary researchers used data from Perseverance’s SuperCam and Mastcam-Z instruments to compare the chemistry and reflectance spectra of the float rocks with deeply weathered paleosols (ancient soils) and hydrothermal kaolin deposits from Earth’s geological record. Aluminum and titanium enrichments coupled with depletion of iron and magnesium are unlike hydrothermal deposits and instead comparable to bleached horizons of paleosols that formed under high rainfall during past greenhouse climates on Earth. These rocks therefore likely represent some of the wettest intervals of Mars’ history.
Mastcam-Z landscape and multispectral images of light-toned float rocks atop the Jezero crater Margin Unit near the Hans Amundson Memorial workspace (Sol 912) demonstrating the spectral diversity of this material. Image credit: Broz et al., doi: 10.1038/s43247-025-02856-3.
“Elsewhere on Mars, rocks like these are probably some of the most important outcrops we’ve seen from orbit because they are just so hard to form,” said Dr. Briony Horgan, a long-term planner on the Perseverance mission and a researcher at Purdue University.
“You need so much water that we think these could be evidence of an ancient warmer and wetter climate where there was rain falling for millions of years.”
“Tropical climates like rainforests are the most common environments to find kaolinite clay on Earth,” added Dr. Adrian Broz, a postdoctoral researcher at Purdue University.
“So when you see kaolinite on a place like Mars, where it’s barren, cold and with certainly no liquid water at the surface, it tells us that there was once a lot more water than there is today.”
The kaolinite fragments, which range from pebbles to boulders, are the latest small pieces to the larger debate regarding Mars’ climate billions of years ago.
Initial examinations by the SuperCam and Mastcam-Z instruments were used to compare the kaolinite to similar rocks found on Earth.
The Martian fragments could offer significant insight into not only the planet’s past environmental stages, but also how Mars came to its current barren state.
“Kaolinite also carries its own mystery,” Dr. Horgan noted.
“There is no major outcropping nearby where the light-colored rocks could have originated despite being scattered throughout the mission path Perseverance has followed since landing at the Jezero crater in February 2021.”
“The crater used to contain a lake about twice the size of Lake Tahoe.”
“They’re clearly recording an incredible water event, but where did they come from?”
“Maybe they were washed into Jezero’s lake by the river that formed the delta, or maybe they were thrown into Jezero by an impact and they’re just scattered there. We’re not totally sure.”
Satellite imagery has spotted large outcroppings of kaolinite in other areas of Mars.
“But until we can actually get to these large outcroppings with the rover, these small rocks are our only on-the-ground evidence for how these rocks could have formed,” Dr. Horgan said.
“And right now the evidence in these rocks really points toward these kinds of ancient warmer and wetter environments.”
Mastcam-Z and SuperCam observations of the hydrated class of aluminum-rich float rocks at Jezero Crater, Mars. Image credit: Broz et al., doi: 10.1038/s43247-025-02856-3.
The researchers compared the Martian kaolinite samples examined by Perseverance with rock samples found in locations near San Diego, California, and in South Africa. The rocks from the two planets were a close match.
Aside from a rain-heavy tropical climate, kaolinite on Earth also forms in a hydrothermal system when hot water is leaching the rock.
But that process creates a different chemical signature in the rock than leaching at lower temperatures by rain over thousands to millions of years.
The scientists used datasets from three different sites to compare the hydrothermal leaching scenario to the Mars rocks.
Rocks on Mars like the kaolinite are a similar time capsule, potentially holding information from billions of years ago about the history of environmental conditions on the planet.
“All life uses water. So when we think about the possibility of these rocks on Mars representing a rainfall-driven environment, that is a really incredible, habitable place where life could have thrived if it were ever on Mars,” Dr. Broz said.
The team’s paper was published in the journal Communications Earth & Environment.
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A.P. Broz et al. 2025. Alteration history of aluminum-rich rocks at Jezero crater, Mars. Commun Earth Environ 6, 935; doi: 10.1038/s43247-025-02856-3
